Internal combustion engines are well known and widely used for propulsive power, electrical power generation, gas compression, liquid and gas transfer, and in various industrial applications. In a conventional four-cycle or two-cycle operating scheme, a mixture of fuel and air is combusted within an engine cycle to produce a rapid pressure rise and induce linear travel of a piston, ultimately rotating a crankshaft to provide torque for various purposes. Spark-ignited engines typically employ a liquid petroleum distillate fuel such as gasoline, or various gaseous fuels in the nature of natural gas, methane, propane, and various mixtures such as biogas, landfill gas, and mine gas. Compression-ignition engines typically utilize fuels such as diesel distillate fuel, biodiesel, and still other liquid fuels. In recent years, there has been significant interest in development of engines and operating strategies that are flexible with regard to fuel utilization. Fuel prices can be fairly dynamic and, moreover, certain fuels that have realized relatively increased abundance in recent years, such as natural gas, can have desirable combustion or emissions properties which are sought to be exploited.
One type of engine design that allows for operation with different fuel types combines both a diesel distillate fuel and natural gas. Diesel alone is relatively easy to compression ignite, but can produce undesired emissions. Where natural gas is used as a fuel in a diesel engine, without modification the mixture of natural gas and air can fail to ignite, knock, or have combustion stability problems. Various strategies have been developed that predominantly burn natural gas while using a relatively smaller amount of diesel fuel as a so-called pilot fuel. The diesel pilot fuel can ignite to in turn ignite the natural gas, offering relative predictability and reliability in the timing and manner of ignition, and otherwise combining certain advantages of both fuel types. One example of such an engine is known from U.S. Pat. No. 6,032,617 to Willi et al.
The term “substitution” or “substitution ratio” is commonly used to describe the relative contributions of diesel fuel and gaseous fuel in a dual fuel engine at any one time, and can be understood generally as the extent to which gaseous fuel is substituted for what would otherwise be diesel fuel in a single fuel liquid fuel engine. In certain dual fuel engines, particularly at relatively high levels of substitution, combustion events can occur that drive cylinder pressures above a maximum limit, typically based upon the capability or tolerance of the hardware to withstand pressure magnitude and pressure impulses. These relatively extreme pressure events can occur for a variety of reasons, including oil droplets present in the combustion chamber, variation in the composition of the gaseous fuel or gaseous fuel blend, under-delivery or over-delivery of gaseous fuel, temperature variation, or still other factors.